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Yeung Man Hau. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1998. / Includes bibliographical references (leaves 52-53). / Text in English; abstract also in Chinese. / Yeung Man Hau. / Chapter Chapter 1 --- Introduction / Chapter 1. --- Background of solidification --- p.1 / Chapter 1.1 --- The driving force for solidification / Chapter 1.2 --- Capillarity effect (or Gibbs-Thomson effect) / Chapter 2. --- Nucleation --- p.3 / Chapter 3. --- Growth --- p.4 / Chapter 3.1 --- Constrained growth and unconstrained growth / Chapter 3.2 --- Directional solidification / Chapter 4. --- Growth of pure substances --- p.6 / Chapter 4.1 --- Metals / Chapter 4.2 --- Stability of planar S/L interface / Chapter 4.3 --- Non-metals / Chapter 5. --- Solidification of single-phase binary alloys --- p.7 / Chapter 5.1 --- Equilibrium solidification / Chapter 5.2 --- Constitutional undercooling / Chapter 5.3 --- Stability of planar S/L morphology / Chapter 5.4 --- Minimum scale of perturbation in directional growth / Chapter 5.5 --- Development of growth morphology / Chapter 5.6 --- Growth rate of cell/dendrite tip / Chapter 5.7 --- Arm spacing and coarsening / Chapter 6. --- Solidification of binary eutectic alloys --- p.11 / Chapter 6.1 --- Classification / Chapter 6.2 --- Growth of lamellar eutectics / Chapter 6.3 --- Stability of planar morphology / Chapter 6.4 --- Coupled zone (Competitive growth of eutectic and dendrites) / Chapter 6.5 --- Off-eutectic solidification / Chapter 7. --- Solidification of ternary eutectic alloys --- p.14 / References --- p.16 / Figures --- p.17 / Chapter Chapter 2 --- Experimental Methods / Chapter 1. --- Fused silica tube cleaning --- p.37 / Chapter 2. --- Alloy preparation --- p.37 / Chapter 3. --- Undercooled specimen preparation --- p.38 / Chapter 4. --- Specimen examination --- p.38 / Chapter 5. --- TEM sample preparation --- p.39 / References --- p.40 / Figures --- p.41 / Chapter Chapter 3 --- Solidification of Undercooled Molten Pd60 .5Cu25Si14.5 Alloy / Chapter 1. --- Introduction --- p.44 / Chapter 2. --- Experimental --- p.46 / Chapter 3. --- Results --- p.46 / Chapter 3.1 --- Thermal profiles / Chapter 3.1.1 --- Temperature-time chart plotter (plotter) / Chapter 3.1.2 --- Differential thermal analysis (D TA) / Chapter 3.2 --- Microstructures / Chapter 3.2.1 --- Effect of undercooling on the microstructure / Chapter 3.2.2 --- Effect of quenching after 1st exothermic peak on the microstructure / Chapter 3.2.3 --- Effect of annealing at the onset temperature of 1st exothermic peak on the microstructure / Chapter 3.2.4 --- Effect of using slower cooling rate on the microstructure / Chapter 4. --- Discussions --- p.50 / Chapter 5. --- Conclusion --- p.51 / References --- p.52 / Figures --- p.54

by Chan Kim Wai. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1997. / Includes bibliographical references. / by Chan Kim Wai. / Chapter Chapter I --- Introduction / Chapter 1.1 --- Rapid solidification / Chapter 1.1.1 --- Rapid quenching --- p.1-1 / Chapter 1.1.2 --- Undercooling --- p.1-2 / Chapter 1.2 --- Grain refinement / Chapter 1.2.1 --- What is grain refinement? --- p.1-5 / Chapter 1.2.2 --- Previous results in grain refinement / Chapter 1.2.2.1 --- Pure metals (or dilute alloys) --- p.1-5 / Chapter 1.2.2.2 --- Alloys --- p.1-9 / Chapter 1.2.2.3 --- Semiconductor --- p.1-10 / Chapter 1.2.3 --- Critical crystal growth velocity V* --- p.1-11 / Chapter 1.2.4 --- Proposed models to grain refinement / Chapter 1.2.4.1 --- Dynamic nucleation and cavitation --- p.1-12 / Chapter 1.2.4.2 --- Remelting (melt-back) --- p.1-14 / Chapter 1.2.4.3 --- Interdendritic fluid flow --- p.1-15 / Chapter 1.2.5 --- Volumetric manifestation of grain refinement --- p.1-15 / Chapter 1.3 --- Aim of this project --- p.1-16 / References / Figures / Chapter Chapter II --- Experimental / Chapter 2.1 --- Pure palladium / Chapter 2.1.1 --- Sample preparation and procedure --- p.2-1 / Chapter 2.1.2 --- Limitation and choice of flux --- p.2-2 / Chapter 2.1.3 --- High temperature furnace --- p.2-3 / Chapter 2.1.4 --- Measurement of specific volume / Chapter 2.1.4.1 --- Theory --- p.2-4 / Chapter 2.1.4.2 --- Setup --- p.2-5 / Chapter 2.1.5 --- Observing internal morphology --- p.2-5 / Chapter 2.2 --- Palladium with insoluble impurity / Chapter 2.2.1 --- Choice of insoluble impurities --- p.2-6 / Chapter 2.2.2 --- Sample preparation --- p.2-7 / References / Figures / Chapter Chapter III --- Results and Discussion / Results / Chapter 3.1 --- Pure palladium / Chapter 3.1.1 --- Specific volume --- p.3-1 / Chapter 3.1.2 --- Grain structure and internal voids --- p.3-2 / Chapter 3.2 --- Palladium with insoluble impurity / Chapter 3.2.1 --- Pinning effect of insoluble impurities --- p.3-3 / Chapter 3.2.2 --- Pd-Ni-S system / Chapter 3.2.2.1 --- Grain refinement in Pd99.9Ni-S)0.1 --- p.3-4 / Chapter 3.2.2.2 --- Change of ΔT* with addition of sulfur --- p.3-5 / Chapter 3.2.2.3 --- Internal voids --- p.3-5 / Discussion / Chapter 3.3 --- Dynamic nucleation of Pd-Ni-S system --- p.3-6 / Chapter 3.4 --- Void formation of pure palladium and Pd-Ni-S --- p.3-6 / Chapter 3.5 --- Grain refinement and specific volume --- p.3-7 / Reference / Figures

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The main aim of this work is to investigate heterogeneous nucleation of solidification of metals and alloys by a combination of differential scanning calorimetry and transmission electron microscopy using a newly modified entrained particle technique. Attention is focused on investigating (a) heterogeneous nucleation of Cd, In and Pb particle solidification by Al in rapidly solidified Al-Cd, Al-In and Al-Pb binary alloys; (b) effects of various ternary additions such as Mg, Ge and Si on heterogenous nucleation of solidification of Cd and Pb solidification by Al; (c) heterogenous nucleation of solidification of Si by solid Al in hypoeutectic Al-Si alloys. In addition, the melting behaviour of Cd, In and Pb particles embedded in an Al matrix is investigated. The rapidly solidified microstructures of melt spun Al-Cd, Al-In and Al-Pb alloys consist of faceted 5-200nm diameter Cd, In and Pb particles homogeneously distributed throughout an Al matrix. Cd particles exhibit an orientation relationship with the Al matrix which can be described as {111}_Al//{0001}_Cd and andlt;110andgt;_Al//andlt;112and#773;0andgt;_Cd, and In and Pb particles exhibit a near cube-cube and cube-cube orientation relationship with the Al matrix respectively. Cd, In and Pb particles embedded in the Al matrix exhibit distorted truncated octahedral or truncated octahedral shapes surrounded by {111}_Al and {100}_Al facets. The solid Al-solid Cd, solid Al-solid In surface energy anisotropies are constant over the temperature range between room temperature and Cd and In melting points respectively. The solid Al-liquid Cd and solid Al-liquid In surface energy anisotropies decrease with increasing temperature above Cd and In melting points. Solidification of Cd, In, Pb particles embedded in an Al matrix is nucleated catalytically by the surrounding Al matrix on the {111}_Al faceted surfaces with an undercooling of 56, 13 and 22K and a contact angle of 42°, 27° and 21° for Cd, In and Pb particles respectively. Addition of Mg to Cd particles embedded in Al increases the lattice disregistry across the nucleating plane, but decreases the undercooling before the onset of Cd(Mg) particle solidification. Addition of Ge to Al decreases the lattice disregistry across the nucleating plane, but increases the undercooling before the onset of Pb particle solidification embedded in the Al(Ge) matrix. These results indicate that chemical interactions dominate over structural factors in determining the catalytic efficiency of nucleation solification in Al-Cd-Mg and Al-Pb-Ge alloys. Contact between Si precipitates and Pb particles embedded in an Al matrix decreases the undercooling before the onset of Pb particle solidification. The equilibrium melting point of Cd particle in the melt spun Al-Cd alloy is depressed because of capillarity, and the depression of equilibrium melting point increases with decreasing particle size. In the melt spun Al-In and Al-Pb alloys, however, most of the In and Pb particles embedded within the Al matrix grains are superheated, and the superheating increases with decreasing particle size. The heterogeneous nucleation temperature for Si solidification by Al depends sensitively on the purity of the Al. Na and Sr additions have different effects on the Si nucleation temperatures. With an Al purity of 99.995%, Na addition increases the Si nucleation temperature, while Sr addition does not affect or decreases the Si nucleation undercooling, depending on the amount of Sr addition. The solidified microstructure of liquid Al-Si eutectic droplets embedded in an Al matrix is affected by the Si nucleation undercooling. With low Si nucleation undercooling, each Al-Si eutectic liquid droplet solidifies to form one faceted Si particle, however, with high Si nucleation undercooling, each Al-Si eutectic liquid droplet solidifies to form a large number of non-faceted Si particles embedded in Al.

edited by N. J. Grant and B. C. Giessen. Section I. / Includes bibliographical references and index.

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Orientadores: Amauri Garcia, Noé Cheung / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-16T22:32:29Z (GMT). No. of bitstreams: 1 Bertelli_Felipe_M.pdf: 6382019 bytes, checksum: 7bc62ea83b7e721ef82e5669d236559f (MD5) Previous issue date: 2010 / Resumo: Neste trabalho, o software ANSYS, baseado no Método dos Elementos Finitos, é adaptado para a simulação tridimensional do fluxo de calor no processo de refusão superficial a laser. A análise numérica é validada com resultados simulados por outros modelos existentes na literatura para casos de refusão superficial a laser de alumínio puro e com resultados simulados e experimentais de uma liga Al-5%Ni. Ensaios experimentais próprios foram realizados em amostras de uma liga Al-1,5%Fe, utilizando um laser à fibra dopado com Itérbio, com potência máxima disponível de 2 kW. Para efeito comparativo, as trilhas foram feitas variando-se valores de velocidade de deslocamento do feixe laser para um mesmo valor de potência. Observou-se que a microestrutura tanto do substrato quanto da zona tratada apresentou morfologia tipicamente celular. As microestruturas resultantes dos tratamentos a laser foram analisadas através de microscopia eletrônica de varredura, sendo observados espaçamentos celulares extremamente refinados na área tratada a laser refletindo no aumento significativo da dureza confirmado por ensaios de microdureza Vickers. Uma técnica de dissolução parcial das amostras tratadas a laser foi aplicada para evidenciar os intermetálicos no substrato e na região tratada a laser, mostrando a modificação da redistribuição dos intermetálicos no interior da poça fundida e dando indicações de aumento da resistência à corrosão na região tratada / Abstract: In this work, the software ANSYS, based on the Finite Element Method, is adapted to simulate the three-dimensional heat flux during the laser remelting surface treatment. The numerical analysis is validated against theoretical results furnished by other models from the literature for laser surface remelting of aluminum and against theoretical and experimental results of Al-5wt%Ni alloy samples. Laser remelting experiments with Al-1,5%wtFe samples have been carried out by using a 2kW Yb fiber laser. For comparative effects, the laser tracks were performed with different laser beam velocities for a fixed value of power. It was observed that both the substrate and the treated region had a typical cellular morphology. The microstructures resulting from the laser treatment were analyzed by using electron scanning microscopy and very refined cell spacing has been observed, which can induce a significant hardness increase confirmed by Vickers microhardness tests. A partial dissolution technique has been performed to foreground the intermetallics at the substrate and at the laser treated zone, showing the intermetallics redistribution inside the molten pool and giving indications of increased corrosion resistance on the treated region / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Mecânica

The primary focus of the present work is the development of macro-models for numerical simulation of binary alloy solidification processes, consistent with microscopic phase-change considerations, with a particular emphasis on capturing the effects of non-equilibrium species redistribution on overall macrosegregation behaviour. As a first step, a generalised macroscopic framework is developed for mathematical modelling of the process. The complete set of equivalent single-phase governing equations (mass, momentum, energy and species conservation) are solved following a pressure-based Finite Volume Method according to the SIMPLER algorithm. An algorithm is also developed for the prescription of the coupling between temperature and the melt-fraction. Based on the above unified approach of solidification modelling, a macroscopic numerical model is devised that is capable of capturing the interaction between the double-diffusive convective field and a localised fluid flow on account of solutal undercooling during non-equilibrium solidification of binary alloys. Numerical simulations are performed for the case of two-dimensional transient solidification of Pb-Sn alloys, and the simulation results are also compared with the corresponding experimental results quoted in the literature. It is observed that non-equilibrium effects on account of solutal undercooling result in an enhanced macrosegregation. Next, the model is extended to capture the effects of dendritic arm coarsening on the macroscopic transport phenomena occurring during a binary alloy solidification process. The numerical results are first tested against experimental results quoted in the literature, corresponding to the solidification of an Al-Cu alloy in a bottom-cooled cavity. It is concluded that dendritic arm coarsening leads to an increased effective permeability of the mushy region as well as an enhanced eutectic fraction of the solidified ingot. Consequently, an enhanced macrosegregation can be predicted as compared to that dictated by shrinkage-induced fluid flow alone. For an order-of-magnitude assessment of predictions from the numerical models, a systematic approach is subsequently developed for scaling analysis of momentum, heat and species conservation equations pertaining to the case of solidification of a binary mixture. A characteristic velocity scale inside the mushy region is derived, in terms of the morphological parameters of the two-phase region. A subsequent analysis of the energy equation results in an estimation of the solid layer thickness. It is also shown from scaling principles that non-equilibrium effects result in an enhanced macro-segregation compared to the case of an equilibrium model For the sake of assessment of the scaling analysis, the predictions are validated against computational results corresponding to the simulation of a full set of governing equations, thus confirming the trends suggested by the scale analysis. In order to analytically investigate certain limiting cases of unidirectional alloy solidification, a fully analytical solution technique is established for the solution of unidirectional, conduction-dominated, alloy solidification problems. The results are tested for the problem of solidification of an ammonium chloride-water solution, and are compared with those from existing analytical models as well as with the corresponding results from a fully numerical simulation. The effects of different microscopic models on solidification behaviour are illustrated, and transients in temperature and heat flux distribution are also analysed. An excellent agreement between the present solutions and results from the computational simulation can be observed. The generalised numerical model is subsequently utilised to investigate the effects of laminar double-diffusive Rayleigh-Benard convection on directional solidification of binary fluids, when cooled and solidified from the top. A series of experiments is also performed with ammonium chloride-water solutions of hypoeutectic and hypereutectic composition, so as to facilitate comparisons with numerical predictions. While excellent agreements can be obtained for the first case, the second case results in a peculiar situation, where crystals nucleated on the inner roof of the cavity start descending through the bulk fluid, and finally settle down at the bottom of the cavity in the form of a sedimented solid layer. An eutectic solidification front subsequently progresses from the top surface vertically downwards, and eventually meets the heap of solid crystals collected on the floor of the cavity. However, comparison of experimental observations with corresponding numerical results from the present model is not possible under this situation, since the associated transport process involves a complex combination of a number of closely interconnected physical mechanisms, many of which are yet to be resolved. Subsequent to the development of the mathematical model and experimental arrangements for macroscopic transport processes during an alloy solidification process, some of the important modes of double-diffusive instability are analytically investigated, as a binary alloy of any specified initial composition is directionally solidified from the top. By employing a close-formed solution technique, the critical liquid layer heights corresponding to the onset of direct mode of instability are identified, corresponding two a binary alloy with three different initial compositions. In order to simulate turbulent transport during non-equilibrium solidification processes of binary alloys, a modified k-8 model is subsequently developed. Particular emphasis is given for appropriate modelling of turbulence parameters, so that the model merges with single-phase turbulence closure equations in the pure liquid region in a smooth manner. Laboratory experiments are performed using an ammonium chloride-water solution that is solidified by cooling from the top of a rectangular cavity. A good agreement between numerical and experimental results is observed. Finally, in order to study the effects of three-dimensionality in fluid flow on overall macrosegregation behaviour, the interaction between double-diffusive convection and non-equilibrium solidification of a binary mixture in a cubic enclosure (cooled from a side) is numerically investigated using a three-dimensional transient mathematical model. Investigations are carried out for two separate model systems, one corresponding to a typical metal-ally analogue system and other corresponding to an actual metal-alloy system. As a result of three-dimensional convective flow-patterns, a significant solute macrosegregation is observed in the transverse sections of the cavity, which cannot be captured by two-dimensional simulations.